acta physica slovaca

Abstract: The multiple exchange of virtual photons emitted by heavy ions with the created lepton pairs are considered. The main attention is devoted to the case when each of the ions emits two virtual photons. We have found rather complicated expression for the ``effective'' Green function of intermediate lepton state. The explicit result for the amplitude and the relevant cross section are obtained for the case when the transferred momenta to the ions are small in comparison with the transversal to beam axes  momenta of created electron and positron. We found the double logarithmical enhancement of the amplitude under consideration, which disappears when the number of emitted photons by each ion exceed two.

Abstract: We are preparing a new experimental program at the DA$Phi$NE facility of LNF to measure X-ray transitions in kaonic deuterium and helium. The hadronic shift and the width of the atomic ground state are sensitive quantities for studying the low energy antikaon-nucleon interaction. Combined with the results from our kaonic hydrogen experiment DEAR this will allow to determine the isospin-dependent scattering lengths. The new kaon-trigger and fast X-ray detectors will provide a suppression of background of more than two orders of magnitude, and thus allow to measure kaonic deuterium X-rays for the first time ever.

Abstract: Recent results obtained by the KLOE experiment operating at DA$Phi$NE, the Frascati $\phi$-factory, are presented. From a sample of about $10^9\ \phi$-mesons produced at \DA$\Phi$NE, we select $K_L$\ and $K_S$ tagged mesons. We present the preliminary results on the search for $K_S -> 3 \pi^{0}$, the branching ratio of the decay $K_S -> \pi e\nu$, and on the major $K_L$ branching ratios, including the semileptonic decays relevant for the $|V_us|$ determination. A preliminary measurement of the $K_L$ lifetime, using $K_L -> \pi^0\pi^0\pi^0$ decays, is also given.

Abstract: The problem of electric and magnetic polarizabilities of pions is illustrated and their extraction from dispersion relations with one subtraction is given. Unambiguous determination of $\sigma$ meson parameters and the decay of $\sigma$ meson into two photons are carried out in order to determine the most realistic values of sum and difference of pion polarizabilities.

Abstract: The HERMES experiment has measured double spin asymmetries in the cross section for deep-inelastic scattering of longitudinal polarized positrons off longitudinal polarized hydrogen and deuterium targets. From these asymmetries, based on inclusive and semi-inclusive measurements, polarized quark distribtions were extracted as a function of $x$. Single-spin azimuthal asymmetries in semi-inclusive pion production were measured by the HERMES experiment for the first time, with a transversely polarized hydrogen target. Two different sine-dependencies were extracted which can be related to the quark transversity distribution $h^q_1(x)$ and the Sivers function $f^{\perp}_{1T} (x)$.

Abstract: The searches for the Higgs boson(s) of the Standard Model and its Minimal Supersymmetric extension with the CMS and ATLAS detectors at the LHC are discussed.

Abstract: The hadronic contribution to the vacuum polarization in the nonperturbative regime of QCD is a crucial issue for the interpretation of the recent measurements of the anomalous magnetic moment of positive and negative muons in Brookhaven to allow for a precision test of the standard model. The present status of the hadronic contribution is reviewed with particular emphasis on recent results with the general purpose detector KLOE at the electron positron collider DA$\Phi$NE in Frascati.

Abstract: The amplitudes of hadron-hadron forward elastic scattering at high energies are investigated on the basis of analyticity and crossing-symmetry. The universal uniformizing variable for them is proposed. The total cross sections for hyperon-proton scattering are predicted. They are consistent with experimental data.

Abstract: An experimental program is conducted by the international collaboration DEAR at Laboratori Nazionali di Frascati (LNF) to measure the strong interaction induced shift and width of the kaonic hydrogen 1s atomic state via X-ray spectroscopy with unprecedented precision. The features of this experiment as well as the performance are described. Preliminary results of the analysis of the kaonic hydrogen X-ray data and a comparison with other experimental and theoretical results are presented. An outlook to the future and perspectives of the ongoing experimental program on kaonic atoms at LNF is given.

Abstract: Meson mass spectrum indicates the existence of a degenerate chiral nonet in the energy region around $1.4$ GeV with a slightly inverted spectrum with respect to a $\overline{q}q$ nonet. Based on the "natural" linear rising of the mass of a hadron with the number of constituent quarks we study the possibility that scalar and pseudoscalar mesons below $1.5$ GeV actually come from the mixing of normal $\overline{q}q$ states with $\overline{q}\overline{q}qq$ states, the latter forming a chiral nonet  chiral symmetry realized directly. We find that the mass spectrum of mesons below $1.5$ GeV is consistent with this picture. In general, pseudoscalar states arise as mainly $\overline{q}q$ states but scalar states turn out to be strong admixtures of $\overline{q}q$ and tetraquark states.

Abstract: The hadronic final state has been investigated in inclusive neutral current deep inelastic $ep$ scattering with the ZEUS detector at HERA. Multiplicity moments of charged particles were studied in the current region of the Breit frame. The mean charged multiplicity observed for inclusive deep inelastic scattering (DIS) events in the pseudorapidity region $|\eta_{\rm lab}| \leq 1.75$ has been measured as a function of the total energy in the current region and as a function of its invariant mass, $M_{\rm eff}$, for both the current and target regions of the Breit frame.

Abstract: Extensive searches for Higgs bosons and other new phenomena predicted by extensions of the Standard Model have been performed at LEP. A summary is given reviewing the principal aspects and presenting a selection of results.

Abstract: Measurements of the proton structure functions at low $Q^2$ are discussed. An extension of previous $F_2$ measurements to large values of $x$ in the transition region from non-perturbative to perturbative QCD is presented, as well as several determinations of $F_L$.

Abstract: Recent results from H1, ZEUS, HERMES and HERA-B on searches for exotic baryons in ep collisions, eD scattering and pA scattering at HERA are reviewed. Evidence for the production of the strange pentaquark $\Theta^{+}$ and of a narrow anti-charmed baryon decaying to $D^{*-}p$ together with negative results of pentaquark searches at HERA are presented.

Abstract: Recent results from the LEP experiments in the area of Standard Model physics are summarized. Measurements of cross-sections, gauge boson couplings and the mass of the W boson are discussed, and the current limit on the mass of the Higgs boson arising from fitting electroweak data is presented. Measurements of $\as$ and recent results in QCD and two-photon physics are discussed.

Abstract: Measurements of charm and beauty production in $ep$ collisions at a center of mass energy of $\sqrt{s}=318$ GeV performed by the ZEUS and H1 experiments at HERA are presented. Final states containing $D$ mesons are used to identify charm production while events containing muons and jets are used to select beauty enriched data samples. Furthermore, new measurements are presented for charm and beauty, which are based on inclusive lifetime tagging methods. Measurements cover both the photoproduction ($Q^2\sim 0$) and deep inelastic scattering (large $Q^2$) kinematic regimes. Experimental results are compared to QCD predictions.